The paper and paperboard industry is a $39 billion/year industry in the United States. Dating back to the 1800s, paper has been made by a process of taking wood or other fibers in water to form a slurry and filtering and drying these slurries on a continuous press. These slurries have a low solids content of the range of 0.5 to 2% and require large amounts of energy to make the finished product. Such paperboards made on a conventional paper machine, such as a Fourdrinier, have densities of 0.3 g/cc or higher depending on further processing such as calendaring. Such paperboards are widely used for all kinds of packaging applications by various industries.
Over the years, attempts have been made to make lower density papers. U.S. Pat. No. 3,716,449 describes a process for making non-woven webs using a foamed fiber dispersion. The examples describe making filter papers, which are generally known to be weak in strength. U.S. Pat. Nos. 3,871,952 and 4,994,843 focus on improving the process of manufacturing such non-woven products. U.S. Pat. No. 4,443,297 describes a method for making non-woven fibrous webs using a foamable liquid that is deposited onto the web.
An article published in the journal Cellulose (A. Madani, S. Zeinoddini, S. Varahmi, H. Turnbull, A. B. Phillion, J. A. Olson and D. M. Martinez, Feb. 18, 2014) describes a method for making ultra-lightweight paper foams. This article describes a method for making paper with ultra-low density (as low as 0.01 g/cc). The mechanical properties described in the paper, however, are very poor.
Although foam forming has been known and has been practiced for some time, the key challenge that has stopped it becoming mainstream is that the mechanical properties of the resulting foam papers are too weak when the density is lowered.
U.S. Publication No. 2015/0114581 describes a method for making foamed paper using microfibrillated cellulose (MFC). The '581 publication describes using 5 to 40 wt % of MFC along with 60 to 95 wt % of pulp having a fiber length greater than the MFC and achieving densities as low as 0.143 g/cc (bulk density of 7 cc/g). The '581 publication describes that, with foamed paper, the structure becomes bulkier (more porous and low density), which leads to smaller tensile strength values. Although the mechanical properties of the foamed paper containing MFC is described as being improved relative to foamed papers containing no MFC, the relative drop in mechanical properties of the foamed paper compared to the products without foam is very significant. In particular, there is a greater than 50% loss in mechanical property when the density is lowered from 0.5 g/cc to 0.25 g/cc (400 j/m2 at a bulk density of 2 cc/g or density of 0.5 g/cc compared to less than 200 j/m2 at a bulk density of 4 cc/g or density of 0.25 g/cc).
Strengthening additives have been used on the wet end of making paperboard. Such agents include water-soluble polyimines, anionic and cationic polyacrylamides, and cationic starches. Despite this, foamed papers or paperboard with lower density and pulp usage and mechanical properties equivalent to that of normal paper are generally not available.